A complete performance of Richard Wagner's "Tristan and Isolde" is an uncommon thing. In the majority of the world'sopera houses where the passionate music drama is presented,it is nearly always subjected to cuts. The principal exception to this practice is at the Wagner shrine in Bayreuth where cuts are never used.

From time to time other major opera theaters occasionally announce uncut performances, though even some of these have suffered the foolish brief sacrificing of a few measures here, a few more there.

The other source of truly complete performances of "Tristan" these days are the phonograph recordings of the opera, of which four are listed in current Schwann catalogs.

Thus the performance of"Tristan," one of the world's greatest operas, to be given at the University of Maryland on Saturday, beginning at 5 p.m., is of unusual importance, carrying, as it does, the assurance that every note of the opera will be sung and played in the all-American production.

Several weeks ago, conductorEve Queler, who has spent nearly a month at Maryland instructing orchestral players in the special arts and skills of playing the operatic repertoire, said she was hoping to give the "Tristan" complete. Her reservation at that time was based on whether or not the orchestra would be able to master the long work - it runs about four hours and a quarter in playing time, depending on the conductor - sustaining the requisite command of tone throughout.

However, these weeks havebrought the players detailed coaching from distinguished playing artists like Mason Jones, the famous first horn of thePhiladelphia Orchestra, Bert Lucarelli on oboe, and others of similar experience both as performers and teachers. Queler has now sent word that the performance will indeed be complete.

Another factor of major significance is involved when every note of the opera is to be sung. Because "Tristan" is rarely given without cuts, those who regularly sing its principal roles often do not know them in their entirety, butrather usually with the cuts that are most often made. (These cuts, by the way, are also by no means standard. Conductor A will decide to leave out 56 measures of the love duet, while Conductor B will prefer to skip some of King Marke's long scene at the end of Act Two)

In the coming performance, the roles of Brangaene, Kurwenal, Melot and King Marke will be sung by Martha Snoddy, Vern Shinall, Curtis Rayam and Louis Eebherz. Since each of these singers is either entirely new to the role or has sung it only a few times, it will notbe a serious problem for them to learn every measure.

But for Roberta Knie, the Isolde, and Jess Thomas, the Tristan, the situation is different. Thomas has sung Tristan for twodecades, sometimes complete, far more often with cuts. Knie, who has just been signed for Isolde in Chicago in the 1979-80 season, has sung her role less often, but is well acquainted with it.

Saturday's performance is to be in concert form, and Queler had wanted the singers to stand and deliver without benefit of scores in front of them. But, she reported, Knie and Thomas had sent word that if the customary cutswere to be omitted - and there is of course no possibility of a prompter - they would have to use their scores at leastsome of the time.

The reasoning behind this argument is not especially sound, since the difference in singing time is less than 15 minutes. But opera singers are a very special breed and some of them get used to doing things one way and do not like to try another.

Supplementing the full orchestra, singers from the university chorus have been trained by Paul Traver for the substantial assignment that closes Act One, when Tristan's sailors and soldiers on board the ship arejoined by King Marke's followers as the ship reaches Cornwall.

There have not been many performances of "Tristan" in any form in Washington. Two seasons ago the Kansas City Philharmonic, under the direction of Maurice Peress, with Jess Thomas and Eileen Farrell in the title roles, gave scenes from the opera. The National Symphony under Howard Mitchell did something similar over a decade ago, while the orchestra was still playing in Constitution Hall.

That performance hadmoments of unexpected and unwelcome humor. Someone had decided that the tenor singing Tristan should, in the second act, make his entrance rushing down the right-hand aisle of the big hall, then up the stairs and onto the stage where he was to embrace Isolde in front of a strange-looking shack that had been provided for her. (For some still-obscure reason, a red light had been hung in front of her house, but no matter.) Anyway - the tenor came roaring down the aisle, up the stairs, and just as he reached his eagerly awaiting beloved, he tripped and fell flat on the floor at her feet, as theecstatic soprano sang her line, "Dare I believe it?"

Whilenothing like this can conceivably happen in Maryland's Tawes Theater, it would be highly welcome to have the singers move about and perform some minimal action that can quite easily suggest something of the flaming emotions that fill this music drama.

Perhaps it is appropriate to recall that Wagner himself wrote to Mathilde Wesendonck, with whom he was in love while composing the third act: "This 'Tristan' is turning into something terrifying! I'm afraid the opera will be forbidden - unless it is turned into a parody by bad performances. Only mediocre performances can save me!"

Instead ofmediocrity, his music has inspired some of the greatest performances in the history of opera. It will be fascinating to see how well the forces the University of Maryland has assembled can handle the immense challenge they have undertaken.